Cu-Ni/TiO2 Catalysts for HMF & Furfural Conversion

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Article
Support Induced Control of Surface Composition in Cu-Ni/TiO2 Catalysts
Enables High Yield Co-Conversion of HMF and Furfural to Methylated Furans
Bhogeswararao Seemala, Charles M. Cai, Charles E. Wyman, and Phillip Christopher
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 08 May 2017
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Support Induced Control of Surface Composition in CuNi/TiO2 Catalysts Enables High Yield Co-Conversion of
HMF and Furfural to Methylated Furans
Bhogeswararao Seemala,†,‡ Charles M. Cai,†,‡ Charles E Wyman,†,‡ Phillip Christopher,‡,#,ǁ,*
†
Bourns College of Engineering - Center for Environmental and Research Technology (CECERT), University of California, Riverside, California 92507, USA.
‡
Department of Chemical and Environmental Engineering, Bourns College of Engineering,
University of California, Riverside, California 92521, USA.
#
Program in Materials Science, University of California, Riverside, Riverside, California
92521, USA.
ǁ
UCR Center for Catalysis, University of California, Riverside, Riverside, California 92521,
USA.
*Corresponding Author Email: [email protected]
___________________________________________________________________________
Abstract
5-(hydroxymethyl)furfural (HMF) and furfural (FF) have been identified as valuable
biomass-derived fuel precursors suitable for catalytic hydrodeoxygenation (HDO) to produce
high octane fuel additives such dimethyl furan (DMF) and methyl furan (MF), respectively.
In order to realize economically viable production of DMF and MF from biomass, catalytic
processes with high yields, low catalyst costs, and process simplicity are needed. Here, we
demonstrate simultaneous co-processing of HMF and FF over Cu-Ni/TiO2 catalysts,
achieving 87.5% yield of DMF from HMF and 88.5% yield of MF from FF in a one pot
reaction. The Cu-Ni/TiO2 catalyst exhibited improved stability and regeneration compared to
Cu/TiO2 and Cu/Al2O3 catalysts for FF HDO, with a ~7% loss in FF conversion over 4
sequential recycles, compared to a ~50% loss in FF conversion for Cu/Al2O3 and a ~30% loss
in conversion for Cu/TiO2. Characterization of the Cu-Ni/TiO2 catalyst by X-ray
Photoelectron Spectroscopy, Scanning Transmission Electron Microscopy, and H2Temperature Programmed Reduction and comparison to monometallic Cu and Ni on Al2O3
and TiO2 and bimetallic Cu-Ni/Al2O3 catalysts suggest that the unique reactivity and stability
of Cu-Ni/TiO2 derives from support-induced metal segregation in which Cu is selectively
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enriched at the catalyst surface, while Ni is enriched at the TiO2 interface. These results
demonstrate that Cu-Ni/TiO2 catalysts promise to be a catalyst system capable of integrating
directly with a combined HMF and FF product stream from biomass processing to realize
lower cost production of liquid fuels from biomass.
Keywords: furfural, hydroxymethyl furfural, hydrodeoxygenation, bimetallic catalysts,
biofuels, support effects
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1. Introduction
Lignocellulosic biomass is the most abundant and inexpensive renewable resource
that can potentially displace petroleum as a carbon neutral alternative for the production of
fungible liquid transportation fuels and commodity chemicals.1–4 However, the production of
target chemicals and gasoline range fuels from lignocellulosic biomass has been
economically challenging due to the need for multiple processing steps and associated high
product yields required in each step.5,6 Biomass is rich in both C6 (glucan) and C5 (xylan)
polymeric sugars that can be converted by acid-catalyzed dehydration into 5(hydroxymethyl)furfural (HMF) and furfural (FF), respectively, with high yields.7–11 As such,
HMF and FF have been identified as valuable fuel precursors suitable for the production of
dimethyl furan (DMF) and methyl furan (MF), respectively, through selective
hydrodeoxygenation (HDO) (hydrogenation followed by hydrogenolysis). Due to their high
octane numbers, these methylated furans (DMF and MF) have been deemed as valuable
target fuel products from biomass suitable for blending with gasoline.12 DMF and MF can
also be selectively converted into C6 and C5 alcohols as direct gasoline replacements, or
further converted into long chain hydrocarbons for diesel or jet fuel applications by
condensation followed by deoxygenation.13,14 However, catalytic production of DMF and MF
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from biomass derived HMF and FF requires high yields and process simplicity.
Supported noble metal catalysts have been thoroughly studied for HDO of HMF and
FF. To reduce costs and control toxicity from catalyst wastes, heterogeneous base metal
catalysts, particularly first row transition metals such as Ni, Cu, Co, and Fe, are excellent
choices.15–17 Undesired reactions such as decarbonylation and ring hydrogenation are
common when using base metal catalysts (particularly Ni, Co and Fe), and the stability of
these materials under reaction conditions is low at high temperatures (≥ 200 °C).18–20 FF
conversion to MF occurs through hydrogenation to furfuryl alcohol (FOL) as an intermediate
product, followed by hydrogenolysis to MF.21,22 Unwanted side reactions include ring
hydrogenation of MF or FOL to form methyl tetrahydrofuran (MTHF), or tetrahydrofurfural
alcohol (THFOL) and decarbonylation of FF to form furan, see Scheme 1. Cu catalysts
minimize decarbonylation and ring hydrogenation due to their full valence d-band and
effectively hydrogenate FF to FOL at low temperatures (< 200 °C).23–26 However, the low
activity of Cu for H2 activation and stronger interaction of Cu with FF, as compared to FOL,
make conversion of FOL to MF on pure Cu catalysts slow.27–29 Cu-based bimetallic catalysts
have been proposed to overcome the low reactivity of monometallic Cu for hydrogenation
reactions. Various formulations have been proposed, such as Cu-Fe, Cu-Pd, Cu-Cr, and CuNi,26,28,30–34 to increase reactivity or selectivity, but further enhancements in reactivity,
selectivity towards MF, and catalyst stability are needed for economical implementation.35
HMF conversion to DMF is known to occur through hydrogenation and
hydrogenolysis of the alcohol and carbonyl groups, but undesirable products are formed
through decarbonylation and ring hydrogenation reactions, see Scheme 2. Noble metals
catalysts such as Ru, Pt, Pd, and their combination with Cu, Fe and Co in bimetallic catalysts
have been reported for HMF conversion to DMF, although the requirement of Pt group metal
catalysts is economically challenging for an industrial process.36–41 Limited reports exist on
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the successful use of solely first row transitions metals or non-noble metal catalyst systems
for high yield conversion of HMF to DMF.42 Recently, it has been demonstrated that
modifying Cu catalysts with Zn or Co and graphene enabled high selectivity for DMF
production.43 However, in these reports low reactivity43 and stability,42 or expensive catalyst
formulations (graphene)43 limit their commercial viability. Bi-functional Ni catalysts
combining dispersed Ni species and acidic supports have also been demonstrated for HMF
conversion to DMF with high yields, although these reports are limited by catalyst stability44
or require high H2 pressures.45
It is clear that base metal catalysts are most suitable for HDO of FF and HMF to
methyl furans due to their low costs, but limited demonstrations of these catalysts are
reported to achieve high reactivity, selectivity, and stability. Furthermore, it has recently been
demonstrated that high yield co-production of HMF and FF can be achieved directly from
biomass in a single step process using THF as a co-solvent, thereby enabling integrated
downstream catalytic strategies to process a single product stream containing both HMF and
FF to reduce overall processing costs.11 However, most previous reports on HMF and FF
HDO have considered their catalytic conversion separately. It is expected that coupling base
metal catalysts capable of simultaneously converting HMF and FF to methylated furans with
recently developed biomass pretreatment technologies could realize significant cost savings
for an integrated processing strategy that avoids separating biomass sugar streams.
Here we demonstrate ~90% yields, high reactivity, good stability, and re-generatable
behaviour for TiO2 supported Cu-Ni bimetallic catalysts in individual and co-processing of
FF and HMF to MF and DMF, respectively. Detailed characterization and reactivity
comparison of monometallic Cu and Ni and bimetallic Cu-Ni supported on TiO2 and Al2O3
demonstrate that for Cu-Ni/TiO2, core-shell structures form in which Cu is enriched at the
catalyst surface. It is proposed that this structure forms due to strong and preferential
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interactions between Ni and TiO2 that reduce the concentration of Ni at the catalyst surface,
thereby allowing Ni to promote Cu reactivity without compromising selectivity. It is expected
that the use of metal-support interactions to control the exposed metal composition in
bimetallic catalysts should be generally useful for enhancing selectivity, reactivity and
stability in variety of catalytic processes.
2. Materials and Methods
2.1 Synthesis of monometallic Cu and Ni catalysts
In a typical synthesis, copper (II) nitrate trihydrate (Cu(NO3)2 3H2O, Aldrich, purity
99%, CAS: 10031-43-3, New Jersey, USA) was dissolved in 50 mL deionized (DI)-water and
added to 5 g of θ-Al2O3 (Inframat Advanced Materials, Catalogue no: 26R-0804UPG,
Manchester CT 06042, USA) or TiO2 (P25, NIPPON AEROSIL Co., LTD, Evonik, Degussa
GmbH, Batch No. 4161060398) contained in a round bottom flask to obtain a 10 wt%
loading of Cu. The solution was mixed and dried at 80 °C in a rotary evaporator. Supported
Ni catalysts were prepared similarly to Cu catalysts, where nickel (II) nitrate hexahydrate
(Ni(NO3)2 6H2O, Aldrich, purity 99.99%, Louis, MO 63103, USA) was used as a precursor in
desired quantities to achieve 10 wt% loadings on TiO2 and θ-Al2O3. The resulting solids
were dried at 100 °C for 12 hours in an oven and calcined at 450 °C for 5 hours. Prior to
reactivity experiments, catalysts were reduced by a pure H2 flow rate of 50 mL min-1 at 450
°C for 3 hours and cooled to 25 °C under the same environment.
2.2 Synthesis of bimetallic Cu-Ni catalysts
Required amounts of Ni and Cu precursors to achieve 5 wt% loadings of each metal
were mixed simultaneously in 50 mL of DI-water and added to 5 g of TiO2 or θ-Al2O3 in a
round bottom flask.
These materials were then mixed and dried at 80 °C in a rotary
evaporator. The solid was collected and dried at 100 °C for 12 hours in an oven followed by
calcining at 450 °C for 5 hours. Prior to reactivity experiments, catalysts were reduced by
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pure H2 at a flow rate of 50 mL min-1 at 450 °C for 3 hours and cooled to 25 °C under the
same environment.
2.3 Catalyst characterization techniques
2.3.1 X-ray diffraction (XRD): XRD spectra of reduced catalysts were recorded in the 2θ
range of 20 to 90° using an X’pert Pro PANalytical diffractometer equipped with a Nickel
filtered Cu-Kα radiation source.
2.3.2. Surface Area: The total accessible surface area (SBET) of the catalysts was measured by
N2 physisorption using a Micromeritics ASAP 2020 instrument.
2.3.3 Scanning Transmission Electrom Microscopy (STEM): STEM imaging was performed
at 300 kV accelerating voltage on an FEI Titan Themis 300 instrument fitted with X-FEG
electron source, 3 lens condenser system, and S-Twin objective lens. STEM images were
recorded with a Fischione Instruments Inc. M3000 High Angle Annular Dark Field
(HAADF) Detector at a probe current of 0.2 nA, frame size of 2048x2048, dwell time of 15
µsec/pixel, camera length of 195 mm, and convergence angle of 10 mrad. Elemental X-ray
microanalysis and mapping were performed utilizing FEI Super-X EDS system with four
symmetrically positioned SDD detectors of 30 mm2 each resulting in effective collection
angle of 0.7 srad. Elemental maps were collected in STEM mode with beam current of 0.4 to
0.25 nA with 512x512 pixel frame, dwell time of 30 µs, and acquisition time of up to 10
mins. Specimens prepared from suspension in distilled water were deposited on copper grids
coated with a lacey carbon. Average metal particle sizes were measured based on the
diameter of 100 particles from corresponding TEM images each catalyst.
2.3.4 X-ray photoelectron spectroscopy (XPS): XPS characterization was carried out using a
Kratos AXIS ULTRADLD XPS system equipped with an Al Kα monochromated X-ray
source and a 165-mm mean radius electron energy hemispherical analyzer. Vacuum pressure
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was kept below 3×10-9 torr during analysis. Binding energy calibrations were done with
reference to the carbon 1s peak by adjusting spectra to 284.8 eV. Depth profiling experiments
were conducted by Argon sputtering samples for 0, 1, 5, 10, 30 and 60 min with beam
voltage of 4 kV, current of 2.35 A, spot size of 3x3 mm2 and vacuum pressure of 3x10-9 Torr
during acquisition. XPS Peak fitting for Cu and Ni components was optimized for each
support, and parameters of the fit were kept constant. A FWHM of 2 eV (2.15 eV) and a
Gaussian/Lorenzian line shape ratio of 30% (60%) was used for all Cu and Ni peak fitting on
Al2O3 (TiO2). Surface composition of bimetallic Cu/Ni catalysts was calculated using
sensitivity factors of 5.321 and 4.044 for Cu and Ni, respectively.
2.3.5 Temperature Programmed Reduction (TPR): TPR experiments were carried out on a
Micromeritics AutoChem 2920 instrument. In each experiment, 0.1 g of catalyst was placed
in a quartz tube and treated with pure Ar flowing at 30 mL min-1 and 150 °C for 1 hour. A
gas mixture of H2 (10%)–Ar (90%) was passed through the quartz reactor at 25 °C for 1 hour
with a 50 mL min-1 flow rate. The temperature was raised to 800 °C at a linear heating rate of
5 °C min-1. A standard CuO powder was used to calibrate H2 consumption.
2.4 Reactivity measurements: Prior to each reaction, Ni, Cu, and Cu-Ni catalysts were
reduced at 450 °C for 3 hours. Without exposure to air, 0.3 g of reduced catalysts were
transferred into a 100-mL stainless-steel Parr micro bench-top reactor containing 1 g of FF
(Sigma Aldrich, 99.9% pure) with 25 ml of either isopropyl alcohol (Fischer Chemical,
HPLC grade) or 1,4 dioxane (Fisher Chemicals, HPLC Grade) as a solvent. The reactor was
initially flushed with H2 and then pressurized under pure H2 environments. Next, the reactor
temperature was raised to 25–240 °C, and the reaction was conducted for 1–8 hours. An
identical protocol was used for the HMF and FF/HMF co-processing reactions, except in
HMF conversion reactions, 0.5 g of HMF was used as a reactant and for FF/HMF coprocessing reactions, 0.5 g of FF and 0.25 g of HMF were used.
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2.5 Product analysis: Liquid products were analyzed by gas chromatography (Agilent
Technologies 7890A; column: DB-WAX Ultra Inert, 30 m long × 0.320 mm internal
diameter × 0.5 micron) via FID according to the following program: hold for 1 min at 30 °C,
increase from 30 to 100 °C at a ramp rate of 10 °C min-1, 2 min hold at 100 oC, increase from
100 to 250 °C at a ramp rate of 25 °C/min, 0 min hold, increase from 250 to 325 °C at a ramp
rate of 25 °C min-1, and 1 min hold at 325 oC. Mass yields of the final product were
quantified by using calibration curves of standard samples in the gas chromatograph. Mass
balances accounting for >95% of the carbon content were obtained in all experiments.
Reactant conversion and product yield were calculated as follows:
() =
% = 1 −
× 100
ℎ ! ! () × 100
2.6 Catalyst recyclability
0.3 g of freshly reduced catalysts were transferred into a 100 mL stainless-steel Parr reactor
containing 1 g of FF and 25 mL of 1,4-dioxane. The reactor was pressurized with H2 to 25
bar, and the reaction was conducted for 2 hours at 200 °C. The reactor was cooled by quickly
lowering it into a room temperature water bath (25 °C) and then depressurized. The catalyst
was separated by filtration, dried at 100° C for 3 hours, and then reused in four recycle
experiments without reduction or re-activation. Regeneration was executed via calcination at
450 °C for 5 hours followed by reduction with pure H2 at 450 °C for 3 hours.
3. Results
3.1 Catalyst characterization
To explore the influence of support and the formation of bimetallic particles on HDO
performance, monometallic Cu (10% weight loading) and Ni (10% weight loading) catalysts
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and bimetallic Cu-Ni catalysts (5%Cu-5%Ni weight loading) on TiO2 and θ-Al2O3 were
synthesized via an impregnation approach. XRD spectra of reduced monometallic (Cu and
Ni) and bimetallic (Cu-Ni) catalysts are shown in Figure S1. Reduced monometallic Cu
catalysts showed distinct peaks at 43.3° and 50.4° that correspond to the (111) and (200)
reflections of metallic Cu, respectively. The reduced monometallic Ni catalysts exhibited
clear peaks associated with the (111) reflection of metallic Ni at 44.9°, although in the case of
the Ni/Al2O3 catalyst this peak overlapped with Al2O3 reflections. The Cu-Ni/Al2O3 catalyst
exhibited a diffraction peak at 43.9°, and the Cu-Ni/TiO2 catalyst exhibited a diffraction peak
at 44.1° and a shoulder at 44.6°. The existence of diffraction peaks between the Cu (111) and
Ni (111) reflections for the bimetallic catalysts are evidence of the formation of Cu-Ni alloy
phases.46,47 However, we refrain from using these peak positions to draw conclusions about
the nature of the Cu-Ni alloy, as the diffraction peaks are predominantly derived from the
largest particles in each catalyst and are not representative of the composition of all particles
in each sample.
Representative TEM images of the monometallic and bimetallic catalysts are shown
in Figure 1 (a, b) and Figure S2, with corresponding particle size distributions shown Figure
S3 and average particle sizes reported in Table 1. The average metallic particle diameter in all
catalysts was relatively consistent, with values between 4.9 and 9.9 nm. It was observed that
for a given metal composition, the Al2O3 supported catalysts exhibited a ~1-2 nm smaller
average particle diameter compared to the TiO2 supported catalysts. This is likely due to the
~20 m2/g greater total surface area of the Al2O3 supported catalysts compared to the TiO2
supported catalysts, see Table 1. From the TEM analysis we conclude that the active metal
particle sizes in all catalysts are relatively consistent and significant differences in catalyst
performance as a function of support for a given metal composition are not expected to be
derived from particle size effects. Elemental mapping executed for the Cu-Ni/TiO2 and Cu-
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Ni/Al2O3 catalysts shown in Figures 2(b-d) and (f-h) demonstrated that Cu and Ni signals coexisted for all observed metal particles. This is consistent with the XRD results, providing
evidence that bimetallic Cu-Ni particles formed on both Al2O3 and TiO2.
XPS was used to characterize the oxidation state and surface composition of the
monometallic and bimetallic catalysts by analyzing the Cu and Ni 2p3/2 peaks. Because these
experiments were performed ex-situ, the exposure to air when catalysts were transferred from
the reduction reactor to the XPS chamber caused partial metal oxidation. Figure 2(a-d) shows
the Cu 2p3/2 spectra for pre-reduced monometallic Cu and bimetallic Cu-Ni catalysts on
Al2O3 and TiO2. All reduced Cu containing catalysts show signatures of Cu0 with binding
energies of 932.1-932.4 eV and Cu2+ with binding energies of 933.7-934.0, and the associated
shake-up satellite peak at ~943 eV, consistent with literature.48 We note that the Cu1+ peak
was not included in this analysis due to overlap with the position of the Cu0 peak, although
this is not expected to significantly influence our conclusions.49,50 Comparing the
monometallic Cu catalysts on TiO2 and Al2O3, we observed that the relative fraction of Cu0
species, with respect to Cu2+, is lower on TiO2 (52.2% for TiO2 versus 62.4% for Al2O3,
Table 1) and that the binding energy of Cu0 is shifted down from 932.4 eV for TiO2 to 932.1
eV for Al2O3. The increased fraction of oxidized Cu2+ and the shift in energy of the Cu0 peak
on the TiO2 support were likely caused by Cu0 catalyzed reduction of TiO2, with subsequent
formation of CuOx species and charge transfer from Cu0 to CuOx or TiO2.48,51 For CuNi/TiO2, the relative fraction of Cu0 compared to Cu2+ increased to values consistent with
those observed for Cu/Al2O3 and Cu-Ni/Al2O3, and the binding energy of the Cu0 species also
shifted down slightly, suggesting weaker interactions between Cu and TiO2 in Cu-Ni/TiO2
compared to Cu/TiO2.
In Figure 2(e)-(h), the Ni 2p3/2 XPS spectra is shown for monometallic Ni and
bimetallic Cu-Ni supported on Al2O3 and TiO2. All supported Ni catalysts showed three
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peaks between 852.2 to 853.0 eV, 854.5 to 854.8 eV, and 856.2 to 856.6 eV that are assigned
to metallic Ni0, NiO, and Ni(OH)2, respectively.52 Binding energies for the Ni0 2p3/2 peaks
were in the order Ni-Cu/Al2O3 ≈ Ni/Al2O3 > Ni/TiO2 ≈ Ni-Cu/TiO2, with the well-known
strong interactions between Ni and TiO2 driving charge transfer from Ti+3 to Ni d-states.53
The shift in binding energy of the Ni0 2p3/2 peak when comparing the monometallic Ni
catalysts on Al2O3 and TiO2 was more significant than that observed for the Cu0 2p3/2 peak
in the monometallic Cu catalysts (0.7 eV versus 0.3 eV), demonstrating stronger interactions
and increased charge transfer at the Ni/TiO2 interface.
The Cu/Ni surface composition ratio was calculated for bimetallic catalysts after
reduction and calcination by summing all contributions to the Cu and Ni 2p3/2 spectra and
correction for XPS sensitivity factors. For the Al2O3 supported bimetallic catalyst, an almost
equal Cu/Ni surface composition ratio of 48.7/51.3 (Table 1) was observed for the reduced
catalyst, consistent with the equal weight loadings of Cu and Ni, the miscibility of Cu and Ni,
and their expected non-specific interactions with Al2O3.47,54 Interestingly, the surface
composition for the TiO2 supported bimetallic catalyst was significantly enriched in Cu, with
a Cu/Ni ratio of 82.4/17.6. As shown in Table 1 and Figure S4-5, the bimetallic catalyst
surface compositions were almost identical after reduction and calcination. The surface
enrichment of Cu and weaker Cu-TiO2 interactions observed by XPS in the Cu-Ni/TiO2
catalyst compared to monometallic Cu/TiO2, combined with the observed and well-known
stronger interactions between Ni and TiO2 compared to Cu and TiO2, suggest that preferential
interactions between Ni and TiO2 drive formation of core-shell type structures on Cu-Ni/TiO2
catalysts where Cu is preferentially exposed at the surface.
To further substantiate the conclusion that TiO2 induced the formation of core-shell
type structures for the bimetallic catalysts, depth profiling XPS experiments were conducted
for reduced Cu-Ni/TiO2 and Cu-Ni/Al2O3 catalysts by using Ar ion bombardment, Figure 3
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and S6-7. For the Cu-Ni/Al2O3 catalyst an almost constant 50%-50% composition ratio was
observed at all Ar sputtering times, which suggests an even distribution of Cu and Ni
throughout the bimetallic particles. For the Cu-Ni/TiO2 catalyst, the relative Cu composition
decreased from 79.1% to 45.5% as the Ar sputtering time was increased from 0 to 60
minutes. The switch in composition from predominantly Cu to slightly Ni enriched with
increasing Ar sputtering time (and thus depth into the particle) is direct evidence of core-shell
like structure for the TiO2 supported bimetallic catalysts. Because these catalysts are high
surface area materials, a precise relationship between Ar sputtering time and sputtering depth
to provide quantitative compositional structure of the particles would be difficult. Regardless
of this limitation, contrast between the relative Cu and Ni compositions as a function of Ar
sputtering time for the Al2O3 and TiO2 catalysts is strong evidence that TiO2 induced the
formation of core-shell structures where the catalytic surface is Cu enriched and the TiO2
interface is Ni enriched.
To corroborate inferences regarding metal-support interactions and Cu surface
enrichment in Cu-Ni/TiO2, temperature programmed reduction (TPR) spectra were measured
as shown in Figure 4. The amount of H2 consumption in the TPR experiments was calculated
for all samples, showing relatively consistent values between 1.3-1.7 mmol/g catalyst, Table
1, which corresponds to ~8-10% of the catalyst mass. This is in agreement with the nominal
~10% metal weight loading in all samples, suggesting that nominal weight loadings well
represent the actual weight loadings and that most Cu and Ni species were reduced in the
TPR experiments.
Figures 4(a, b) show that the TPR spectra of Cu/Al2O3 exhibited a single reduction
peak at 191 °C, whereas two peaks were observed for Cu/TiO2 at 127 and 192 °C. The low
temperature reduction peak for Cu/TiO2 is attributed to CuOx species directly interacting with
the TiO2 support, whereas the ~190 °C reduction peak is assigned to bulk-like CuOX.48 In the
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case of Ni/Al2O3, Figure 4(c) shows that two reduction peaks were observed at 350 °C and
500 °C. The former is assigned to the reduction of amorphous NiO species, while the latter is
assigned to reduction of crystalline NiO.52 Depending on synthesis procedure and calcination
temperature, non-stoichiometric and stoichiometric Ni-aluminates may also form, although
these species (TPR peaks > 500 oC) were not observed here.55 The TPR spectra of Ni/TiO2 in
Figure 4(d) showed three peaks at 250 °C, 350 °C, and 450 °C. The peaks at 250 °C and 350
°C are assigned to strongly interacting amorphous and crystalline NiO on TiO2, due to their
significant shift down in temperature compared to Ni/Al2O3, and the peak at 450 °C is
assigned to the onset of TiO2 reduction.56 TPR results from the monometallic catalysts are in
agreement with XPS results, providing evidence for significant TiO2 interactions with Cu and
Ni and that this interaction is stronger for Ni-TiO2.
For Cu-Ni/Al2O3, the reduction peaks associated with Cu and Ni in Figure 4(e) both
shifted to lower temperatures, as compared to the monometallic catalysts supported on Al2O3.
This result suggests an intimate interaction between Ni and Cu species, consistent with the
TEM and the XPS analyses that showed similar surface concentrations of Cu and Ni.57,58 For
Cu-Ni/TiO2, the sharp reduction peak observed at 180 °C in Figure 4(f) was assigned to CuOx
reduction, and the very weak reduction peak at 277 °C was assigned to amorphous NiO
reduction. The dominant signature of CuOx reduction coupled with the lack of peak
associated with reduction of CuOx species that are directly interacting with TiO2 (seen
previously at 127 oC for Cu/TiO2) are strong evidence of the core-shell structure for CuNi/TiO2, where Cu is exposed at the surface due to strong Ni-TiO2 interactions.
To summarize catalyst characterization, monometallic Cu and Ni catalysts on Al2O3
and TiO2 exhibited clear signatures of metal-support charge transfer on TiO2, and the strength
of this interaction was greater for Ni compared to Cu. For the bimetallic catalysts, STEM
imaging and elemental mapping showed that all metal particles observed on TiO2 and Al2O3
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contained Cu and Ni, demonstrating the formation of bimetallic particles. For Cu-Ni/Al2O3 it
was observed that the bimetallic particles contained a homogeneous mixture of Cu and Ni
throughout the particles. However, for Cu-Ni/TiO2 strong Ni-TiO2 interactions induced the
formation of core-shell like structures enriched in Cu at the catalytic surface. While it is well
documented that surface segregation in bimetallic catalysts can be driven by reactant
adsorption, the use of preferential metal-support interactions to control surface compositions
in bimetallic catalysts is much less common.59–61
3.2. Catalytic activity
3.2.1 Hydrogenation of furfural: The conversion of FF to MF follows a consecutive reaction
pathway with many parallel reactions that can drive formation of undesired products. In
addition to catalyst composition, it has been observed that reaction selectivity (and ultimately
yields) are sensitive to reaction temperature, time, H2 pressure, and catalyst/reactant loading.
We screened reaction conditions of 180 to 240 °C, 15 to 55 bar H2, and 5 to 30%
catalyst/reactant mass loading ratios to identify the conditions at which both Ni and Cu
metals produced the highest MF yields shown in Figure 5 and Table S1-3. It was identified
that at a temperature of 200 °C, 25 bar H2 pressure and catalyst/reactant loading of 30%,
decarbonylation and ring opening reactions were significantly suppressed and the reactions
on all catalysts were mostly by hydrogenation and hydrogenolysis of FF. All subsequent
reactions were performed at these conditions. It is also worth noting that for initial catalyst
screening, isopropyl alcohol was used as a solvent. However, because of significant solvent
conversion, all subsequent reactivity comparisons with 1,4-dioxane as the solvent due to its
stability at optimized reaction conditions.
The conversion of FF and yield of various products over monometallic Ni and Cu on
Al2O3 and TiO2 catalysts at 1 and 8 (4 for Ni) hour reaction times is shown in Figure 6.
Generally, monometallic Cu catalysts were selective for MF production, while Ni catalysts
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were selective for production of THFOL.62 The Ni catalysts were significantly more reactive
than Cu catalysts, as evidenced by the ~ 1 order of magnitude longer reaction time required
for full FF conversion on Cu. On the Ni/Al2O3 catalyst, FF was completely converted with a
54.5% yield of THFOL and 30.5% yield of MF after 1 hour, and the product selectivity did
not change after 4 hours. The Ni/TiO2 catalyst was less active than Ni/Al2O3, with some FOL
remaining after 1 hour and the product selectivity stabilizing after ~4 hours with 70% yield of
THFOL and ~15% yield of MF. THFOL as the primary product over the Ni catalyst is
consistent with the known strong interactions between the furan ring in FF and Ni surfaces,
which drives ring hydrogenation.62,63 The strong interactions between Ni and TiO2 observed
by XPS and TPR and the enhanced THFOL yields on Ni/TiO2 compared to Ni/Al2O3 indicate
that Ni sites near the TiO2 interface coordinate more selectively with the furan ring in FF
rather than the carbonyl (or subsequently produced alcohol) group, thus promoting ring
hydrogenation rather than carbonyl hydrogenolysis.
Cu catalysts were much less active than Ni catalysts and required ~8 hours to achieve
complete FF conversion, likely due to their weak ability to activate H2.64 Similar to the Ni
catalysts, Cu/Al2O3 was more active than Cu/TiO2. Figure 6 shows that for Cu/Al2O3 41% of
FF was converted to yield 21.5% FOL and 17.2% MF at 1 hour, while for Cu/TiO2, 25.7% of
FF was converted to yield 12.1% FOL and 12.7% MF at 1 hour. Further extending reaction
times to 8 hours resulted in complete FF conversion for both catalysts with similar final MF
yields of 74.9% for Cu/Al2O3 and 75.9 % for Cu/TiO2. THFOL yields were suppressed on Cu
catalysts compared to Ni, due to repulsive interactions between the Cu 3d band and the
aromatic furan ring.25 To more effectively compare FF conversion on the monometallic Cu
catalysts, Figure S8 shows FF conversion and product yields over time on both the Cu/Al2O3
and Cu/TiO2 catalysts. In both cases, the time dependent yield profile of FOL strongly
suggests that this species is an intermediate in the production of MF, as previously
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reported.32,65 There were slight differences in the time dependent yields of FOL and MF for
the Cu/TiO2 and Cu/Al2O3 catalysts, particularly at all times < 6 hours for which higher
yields of FOL are realized on Cu/Al2O3 than on Cu/TiO2. These results suggest that the initial
hydrogenation of FF to FOL occurs more effectively on Cu/Al2O3, whereas the
hydrogenolysis of FOL to form MF occurs more readily on Cu/TiO2. Although Cu/Al2O3 is
more active than Cu/TiO2, small amounts of undesired products such as diols and pentanols
were obtained at an 8 hour reaction time on Cu/Al2O3. Differences in reactivity of the
monometallic Cu catalysts were likely caused by interactions between Cu and the support,
which can change the charge state of the active metal or introduce interfacial reaction
pathways and the inherent acidity of the support.44,45 Comparing Ni and Cu catalysts, it is
clear that Ni promotes ring hydrogenation to form THFOL, while Cu is selective for
hydrogenolysis to form the desired product MF and that the support composition only mildly
influenced catalyst performance.
While the nature of the support induced relatively small changes in the reactivity of
monometallic Cu and Ni catalysts, the support significantly influenced the selectivity and
reactivity of FF conversion for the Cu-Ni bimetallic catalysts, as shown in Figure 7. CuNi/Al2O3 was more active than Cu-Ni/TiO2 with 100% FF conversion achieved in 0.5 hour
reaction time for Cu-Ni/Al2O3, whereas 8 hours was required for complete FF conversion on
Cu-Ni/TiO2. On Cu-Ni/TiO2, MF and FOL were the primary products, with MF selectivity
increasing with reaction time to ultimately achieve a 92.1% yield of MF at 8 hours.
Conversely, the Cu-Ni/Al2O3 catalysts favoured furan ring hydrogenation which resulted in
increasing THFOL yields with subsequent loss of FOL, as reaction time was extended. When
comparing results with Cu-Ni/Al2O3 to those with monometallic Ni and Cu catalysts, both the
reactivity (time for complete FF conversion) and product distribution (primarily THFOL, MF
and at short times FOL) strongly resembled the behaviour of the monometallic Ni catalysts.
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Cu-Ni/TiO2 showed reactivity behaviour that was more similar to Cu, with only MF and FOL
as significant products. However, Cu-Ni/TiO2 exhibited about 2-fold higher conversion at the
same reaction time compared to monometallic Cu/TiO2 and significantly enhanced MF yields
at longer reaction times (92% vs. 75.9%).
Issues previously identified for the application of Cu-based catalysts to FF HDO are
the potential for Cu sintering or carbon deposition affecting stability at reaction conditions.
To examine these effects, recycle and regeneration experiments were performed where four
sequential reactivity experiments were executed without treating the catalyst between
experiments, followed by calcination and reduction of the catalysts prior to a final reactivity
experiment. It is expected that loss in reactivity during the four sequential experiments could
be due to carbon deposits and Cu sintering, while regeneration should predominantly remove
carbonaceous deposits, allowing differentiation of the catalyst degradation mechanisms. As
shown in Figure 8(a), using the Cu/Al2O3 catalyst, FF conversion dropped from 56% to 10%
by the fourth reactivity experiment (R4) and then increased to ~47% after a regeneration step
(R5), whereas MF selectivity was relatively similar in the first (R1) and fifth (R5)
experiment, Figure 8(a). The results are consistent with previous studies of Cu/Al2O3
catalysts showing low stability under FF HDO conditions.66–68 For Cu/TiO2, the FF
conversion decreased from 47% to 18% by R4, but increased to 37% following regeneration
(R5), with a similar selectivity to R1, Figure 8(b). The lower loss of reactivity during the four
sequential experiments (R1 - R4) for Cu/TiO2 compared to Cu/Al2O3 suggests that the
amount of carbonaceous deposits is reduced by the use of TiO2 as a support. However, the
similar change in reactivity compare R1 and R5 for Cu/TiO2 and Cu/Al2O3 suggests that Cu
sintering similarly occurs for both catalysts.
For the Cu-Ni/TiO2 catalyst, FF conversion decreased from 93.5 to 85.1% from R1 to
R4, demonstrating enhanced stability compared to the monometallic Cu catalysts, Figure
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8(c). Regeneration of Cu-Ni/TiO2 restored the MF and FOL yields seen in the initial
experiment, although a small amount of THFOL formation was also observed. The results
demonstrate that Cu-Ni/TiO2 catalysts exhibit significantly increased resistance to reactivity
degradation caused by carbon deposition and sintering compared to Cu/TiO2 and Cu/Al2O3.
However, it was also observed that a combination of exposure to reaction conditions and
regeneration of the Cu-Ni/TiO2 catalyst resulted in the exposure of small amounts of surface
Ni domains that drive ring hydrogenation.
3.2.2. HDO of HMF and co-processing of FF and HMF: Because Cu-Ni/TiO2 showed
excellent reactivity, MF selectivity, and stability for FF conversion, this catalyst was further
tested for reactivity in HMF conversion to DMF and simultaneous co-processing of HMF and
FF to DMF and MF, respectively. The measured time dependent conversion of HMF and
HMF with FF (co-processing) and product yields on Cu-Ni/TiO2 are shown in Figure 9(a)
and 9(b), respectively. HMF conversion to DMF is known to occur through two reaction
pathways.43 In the first pathway, hydrogenolysis of HMF yields methyl furfural (MFF) and
then hydrogenation followed by hydrogenolysis of MFF to DMF, as shown in Scheme 2. In a
second pathway, 2, 5 bis(hydroxymethyl) furan (BHMF) is the intermediate product, and
further hydrogenation of both C-O groups in BHMF results in DMF formation. The
conversion of either BHMF or MFF to DMF typically occurs through methylfurfuryl alcohol
(MFOL), per Scheme 2. However, MFOL is highly reactive and immediately converts into
DMF by hydrogenolysis.45
For HMF conversion over Cu-Ni/TiO2, 95.9% conversion of HMF was observed
within 0.5 hour of reaction, with the primary products being DMF (52.2% yield) and MFF
(30.7% yield). With increasing reaction times, MFF was converted selectively to DMF to
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ultimately result in 84.3% yield of DMF at 8 hour reaction time. Small amounts of MFOL
were also observed as a rapidly converted intermediate in the production of DMF from MFF.
In the co-processing of HMF and FF over Cu-Ni/TiO2, 0.5 g of FF and 0.250 g of
HMF were used as reactants with 0.3 g of catalyst at the same reaction conditions as the HMF
and FF cases. Cu-Ni/TiO2 showed similar activity and methylated furan (MF + DMF) yields
as in individual processing of HMF and FF. Moreover, based on the time dependent yields of
intermediate products (FOL and MFF, for example), the mechanism of methylated furan
production did not change for co-processing of FF and HMF. The maximum yields of MF
(88.5%) and DMF (87.5%) were realized after 8 hours of reaction at very similar yields as for
the individual processing cases.
Summarizing the reactivity results, the monometallic Cu and Ni catalysts showed FF
hydrogenation and HDO reactivity consistent with previous reports, and their reactivity
exhibited minimal sensitivity to support composition.23,65,69 However, the bimetallic Cu-Ni
catalysts showed significant support effects for FF HDO. On Al2O3, the bimetallic catalyst
acted similarly to Ni alone, while on TiO2, the Cu-Ni bimetallic catalyst enhanced rates and
MF selectivity compared to Cu alone. The Cu-Ni/TiO2 catalyst showed good stability,
regenerability, reactivity, and MF selectivity and outperformed monometallic Cu catalysts in
all performance metrics at similar conditions.41,43 Finally, Cu-Ni/TiO2 showed excellent
selectivity toward methylated furans in HMF and FF/HMF co-processing reactions.
4. Discussion
The significant support effect observed for bimetallic Cu-Ni catalysts can be
explained and rationalized through the catalyst characterization studies. Cu-Ni/Al2O3
reactivity studies suggested that the behaviour of the catalyst significantly resembled pure Ni
catalysts, where ring hydrogenation was prevalent. Both TPR and XPS showed evidence of
equal amounts of Cu and Ni exposed at the catalyst surfaces, while the TEM images
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suggested that all catalytic particles contained Ni and Cu. On Ni, the primary η2(C=O) FF
adsorption geometry brings the aromatic ring close to the surface, enabling ring
hydrogenation (Figure 10).44,70,71 Because the ring-surface interaction is only expected at Ni
surface domains that have Ni ensemble sizes large enough to coordinate to the furan ring, or
at Cu-Ni alloy surface structures with electronic structures significantly resembling
monometallic Ni, it can be concluded that Ni-rich domains at the surface of bimetallic Cu-Ni
particles were responsible for a majority of the reactivity of the Cu-Ni/Al2O3 catalyst. The
proposed Cu-Ni bimetallic particle structure on Al2O3 based on the XPS measurements and
reactivity results is shown schematically in Figure 10.
The Cu-Ni/TiO2 catalyst reactivity significantly resembled that of Cu alone, albeit
with enhanced reaction rates and MF selectivity. This result can be explained based on the
depth profiling XPS, TPR, and TEM results, which together suggest that TiO2 supported
bimetallic Cu-Ni particles segregate into core-shell geometries that preferentially expose Cu
at their surface, as shown schematically in Figure 10. On Cu surfaces, the FF ring is repelled
from the surface due to the full valance d-band, forming the η1(O) FF adsorption geometry
and thus, as opposed to Ni surfaces, carbonyl hydrogenation is favored on Cu over ring
hydrogenation. The enhanced reactivity and MF selectivity on Cu-Ni/TiO2 compared to
monometallic Cu catalysts, with no additional ring hydrogenation or decarbonylation, is
primarily attributed to increased rates of H2 dissociation.33,64 The enhanced rates of H2
dissociation but retained Cu selectivity characteristics could occur at so called single atom
surface alloys, where small collections (1-3, essentially small enough so they cannot
coordinate to a furan ring) of Ni atoms are exposed at the Cu surface to allow for enhanced
H2 dissociation rates but not the η2(C=O) FF adsorption geometry.64,72,73 These reactivity
characteristics could also be consistent with a low relative Ni concentration near surface alloy
that electronically or sterically modifies the exposed surface Cu atoms and enables enhanced
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H2 dissociation rates.74,75 Although we cannot differentiate these two mechanisms, it is clear
that Cu was significantly enriched at the surface in Cu-Ni/TiO2 catalysts, yet still modified by
Ni in a manner that enhanced H2 dissociation rates without changes in the inherent preference
of Cu for carbonyl hydrogenolysis over ring hydrogenation (see Figure 10).
The above discussion, combined with the weak influence of the support composition
on the reactivity of the monometallic catalysts, strongly suggest that the primary role of TiO2
in promoting the catalytic properties of the Cu-Ni catalyst is indirect, with Ni driven to the
TiO2 interface and Cu preferentially exposed at the surface. Thus, TiO2 controls the
bimetallic Cu-Ni catalyst surface composition but does not directly participate in the catalytic
process.76,77 A well-known behaviour on TiO2 supported Pt-group metal catalysts is strong
metal support interactions (SMSI), where support reduction (oxygen vacancy formation)
drives TiO2 encapsulation of the active metal particle.78–80 While a metal selective SMSI
encapsulation state that preferentially encapsulates Ni could be imagined, the TPR data
oppose this interpretation. SMSI encapsulation states with TiO2 are typically observed
following > 500 oC reduction treatment. Because minimal Ni reduction peaks were observed
for the Cu-Ni/TiO2 catalysts despite Ni reduction peaks being clearly observable in the NiTiO2 catalyst, Ni is likely buried subsurface in the bimetallic particles prior to SMSI
encapsulation layer formation. This mechanism is further supported by the XPS analysis of
surface composition in the pre-reduced and pre-oxidized Cu-Ni/TiO2 catalysts that show
identical Cu/Ni surface concentration ratios in Table 1. Thus, preferential interactions
between Ni and TiO2 are believed to drive formation of core-shell like particles where Cu is
primarily exposed at the catalyst surface, see proposed structures in Figure 10. We propose
that support induced bimetallic particle segregation may be quite general for reducible oxide
supported bimetallic catalysts, given known metal specific interactions with reducible
supports.81
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The structural properties of Cu-Ni alloys have been extensively studied for their
potential application to a range of reactions.82–90 Bulk Cu-Ni alloys are miscible and
equilibrated into alloy phases when heated above ~400 oC.91 In bulk systems (polycrystalline
films, single crystalline films, and other structures), Cu surface segregation has been reported
to be due to the lower surface energy of Cu in vacuum compared to Ni.
85,92–96
It has further
been demonstrated that the surface segregation is a facet dependent phenomenon that occurs
more predominantly on (100) surface facets compared to (111) surface facets.93,97 Insights
into the structure of bulk Cu-Ni alloys have not translated into a complete understanding of
the structure of supported Cu-Ni alloy nanoparticles, where multiple surface facets are
simultaneously exposed, and the distribution of Cu and Ni is also influenced by interfacial
interactions with the support and environment.
Previous reports of 50%-50% Cu-Ni alloy nanoparticles supported on irreducible
supports (SiO2 and Al2O3) have found that as-synthesized particles show no evidence of Cu
surface segregation.47,54,90,98,99 However, reports of Cu-Ni alloy nanoparticles on reducible
supports (CeO2, ZrO2, MgO), or supports with significant surface defect concentrations
(SBA-15), have shown evidence of Cu enriched surfaces, in agreement with our
findings.49,89,100,101 Thus, while bulk Cu-Ni alloys exhibit Cu surface segregation due to the
decreased surface energy of Cu in vacuum, it seems that for supported Cu-Ni alloy
nanoparticles preferential Ni-support interactions on reducible or defective supports is the
primary driving force for Cu surface segregation. Also, reconstruction of the as-synthesized
catalytic structure under reaction conditions may be expected when significant adsorbatemetal specific interactions exist, for example in a CO atmosphere.90 However based on the
lack of ring hydrogenation observed in our studies for the Cu-Ni/TiO2 catalyst, it can be
concluded that migration of Ni to the catalyst surface is minimal under reaction conditions.
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It is worth comparing our results to recent reports that examined the influence of
modification of Cu catalysts with Ni on reactivity in FF HDO. A surface science approach
was used to compare pure Cu (111) and Ni (111) surfaces with a Ni surface layer on Cu (111)
and a Cu surface layer on Ni (111) for FF hydrogenation.102 It was clearly seen that in both
surface layer configurations, bimetallics enhanced selectivity towards MF formation
compared to monometallics. However, even in the case of Cu monolayers on Ni (111),
significant nonselective decomposition of FF was observed. This result suggests that for the
Cu-Ni/TiO2 catalysts examined here, the Ni concentration in the near surface region is much
lower than for the prior surface science study. A separate study of the influence of Ni
addition at various concentrations to Cu/Al2O3 on FF hydrogenation reactivity and
selectivity33 showed that increasing the Ni loading increased the rate of FF conversion, but
also increased the selectivity towards ring hydrogenation, decarbonylation, and ring opening
products. The introduction of self-assembled organic monolayers to the catalyst lowered the
Ni surface content in Cu-Ni/Al2O3, thereby boosting selectivity and reaction rate for
hydrogenation products (we note that FOL was the main product in these studies, which is
likely caused by the reactions being performed in the gas phase). In this case, it seems that
the use of self-assembled monolayers to control the exposed surface concentration of Ni in
bimetallic Cu-Ni catalysts bears some resemblance to our reported use of TiO2 to control the
Ni surface concentration in bimetallic Cu-Ni catalysts. Although, the Cu-Ni/TiO2 catalysts
reported here enable complete conversion and high yields to MF and are expected to have
enhanced stability compared to the organic functionalized Cu-Ni catalysts reported
previously, thus creating a more scalable approach for controlling Cu surface chemistry for
selective hydrogenation reactions. In addition, it was very recently reported that increasing
the Ni content in Cu-Ni/MgAlO catalysts enhanced FF conversion (>99%), however, FOL
and THFOL were only selective products observed.58
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In addition to considering how TiO2 induced segregation of Cu-Ni catalyst particles
influenced reactivity and selectivity, it is also interesting to explore how this factor may have
influenced stability and catalyst regeneration. In the recycling-regeneration experiments
shown in Figure 8, it was observed that the Cu/TiO2 and Cu/Al2O3 catalysts exhibited similar
performance degradation that is ascribed to carbonaceous deposits on the metal and Cu
sintering. The improved stability of Cu-Ni/TiO2 compared to the monometallic catalysts
during recycle experiments (R1 - R4) is likely due to decreased carbon deposition on the
catalysts driven by increased H2 dissociation rates. Increased stability of the Cu-Ni/TiO2
catalyst compared to the monometallic Cu catalysts when considering R1 and R5 is likely due
to reduced sintering of the active Cu metal. This suggests that Ni serves as an anchoring site
for Cu on TiO2, which enhances catalyst stability and provides a stable platform for
regeneration of the core-shell Cu-Ni structure to that for high MF selectivity and reactivity.
Further reduction of the Ni loading in Cu-Ni/TiO2 catalysts may allow for similar enhanced
reactivity and stability as observed here while also minimizing the surface exposure of Ni
observed with increased time under reaction conditions.
Recently, it has been shown that co-solvent-enhanced lignocellulosic fractionation
(CELF) of raw biomass enables the production of HMF and FF with extremely high yields.11
Separation of HMF and FF in a liquid stream from the remaining lignin can be effectively
achieved through various approaches to yield a combined stream of HMF and FF that is
typically rich in FF. Our demonstration of high yield, single pot conversion of FF and HMF
over stable and regenerable Cu-Ni/TiO2 catalysts opens new possibilities for an efficient and
high yield biomass to fuels conversion process with only a few required process steps. It is
expected that coupling of CELF pretreatment of cellulosic biomass with FF/HMF catalytic
co-processing will enable an effective approach for conversion of raw biomass to high quality
fuels.
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5. Conclusions
In conclusion, our study showed that Cu-Ni/TiO2 is a unique catalytic material that
enabled high yield (~90%) conversion of FF and HMF to methylated furans in either single
or co-processing schemes, results not possible with monometallic Cu and Ni, or Cu-Ni/Al2O3.
The reactivity of Cu-Ni/TiO2 is proposed to result from strong and selective Ni-TiO2
interactions that favored in formation of Cu-shell and Ni-core structure, allowing for high
selectivity in HDO and enhanced reactivity compared to monometallic Cu catalysts.
Furthermore, the strong Ni-TiO2 interactions effectively anchored the bimetallic particles to
the TiO2 support, thereby reducing catalyst degradation via sintering and enabling effective
regeneration. Finally, it is envisioned that a potentially economical biomass to fuel
conversion process can be achieved by coupling CELF pretreatment of raw biomass to
produce high yield liquid streams of HMF and FF with catalytic co-processing using CuNi/TiO2 to high yields of methylated furans.
Associated Content
Supporting Information. Additional catalyst characterization (XRD, TEM, XPS) and
reactivity data is provided in Figures S1-S8 and Tables S1-S3.
Acknowledgements
Authors acknowledge funding support from DOE-EERE BETO Office through Award DEEE0007006. Authors acknowledge Dr. Krassimir N. Bozhilov for assistance with the electron
microscopy performed at the CFAMM at UC Riverside. Dr. Ilkeun Lee is acknowledged for
assistance with the XPS analysis made possible by NSF grant DMR-0958796.
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(94) Sinfelt, J. H; Carter, J. L; and Yates, D. J. C. Journal of Catalysis 1972, 24, 283–296.
(95) Wandelt, K. Physical Review Letters 1981, 46, 1529–1532.
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20–40.
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(98) Mariño, F.; Baronetti, G.; Jobbagy, M.; Laborde, M. Applied Catalysis A: General
2003, 238, 41–54.
(99) Studt, F.; Abild-pedersen, F.; Wu, Q.; Jensen, A. D.; Temel, B.; Grunwaldt, J.;
Nørskov, J. K. Journal of Catalysis 2012, 293, 51–60.
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of Materials Chemistry 2011, 21, 12529–12541.
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(88)
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Table 1. Physicochemical properties of monometallic Cu, Ni and bimetallic Cu-Ni catalysts supported on Al2O3 and TiO2
Sample
Specific
surface area
(SBET, m2/g)
Particle size
distribution
from TEM
(nm)
H2-Uptake
(TPR;
mmol/g)
Cu/Ni Surface Composition
Cu2+/Cu0
Ni2+/Ni0
Cu/Ni
(Cuo+Cu+2)/
(Ni0+Ni+2)
θ-Al2O3
79.7
-
-
-
-
-
TiO2(P25)
54.3
-
-
-
-
-
Cu(10 wt%)/Al2O3
64.0
7.0±1.6
1.29
37.6/62.4
-
-
Cu(10 wt%)/TiO2
43.4
8.0±3.5
1.48
47.8/52.2
-
-
Ni(10 wt%)/Al2O3
64.8
6.4±5.1
1.68
-
52.4/47.7
-
Ni(10 wt%)/TiO2
47.2
9.9±2.6
1.75
-
57.5/42.7
-
Cu(5 wt%)-Ni(5 wt%)/Al2O3
67.4
4.9±1.6
1.62
38.6/61.4
73.7/26.3
48.7/51.3b
Cu(5 wt%)-Ni(5 wt%)/TiO2
45.9
7.5±3.9
1.56
33.1/66.9
57.5/42.5
82.4/17.6b
Cu(5 wt%)-Ni(5 wt%)/Al2O3a
-
-
-
-
-
47.2/52.8
Cu(5 wt%)-Ni(5 wt%)/TiO2a
-
-
-
-
-
82.4/17.6
a
Catalysts were calcined prior to analysis.
Cu/Ni ratio is from XPS shown in Figure 2 and these are consistent with the values in Figure 3.
b
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Figure 1. (a) Representative STEM image of the Cu-Ni/Al2O3 catalyst and associated (b) Cu, (c) Ni, and (d) overlaid Cu/Ni/Al elemental
mapping. (e) Representative STEM image of the Cu-Ni/TiO2 catalyst and associated (f) Cu, (g) Ni, and (h) overlaid Cu/Ni/Ti elemental
mapping.
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932.4
625 (a) Cu/TiO2
500
375
250
400
(b) Cu-Ni/TiO2
350
300
250
933.9
300 (c) Cu/Al2O3
240
180
120
934.0
400 (d) Cu-Ni/Al2O3
320
240
160
946 944 942
932.3
932.1
938
936
934
932
852.2
854.7
300 (f) Cu-Ni/TiO2
290
280
852.2
854.9
240 (g) Ni/Al2O3
200
160
120
852.9
854.7
350 (h) Cu-Ni/Al O
2 3
300
250
200
932.1
933.7
940
854.8
340 (e) Ni/TiO
2
320
300
280
934.0
Intensity (a.u.)
Intensity (a.u.)
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930
866
928
864
862
852.9
860
858
856
854
852
850
848
Binding Energy (eV)
Binding Energy (eV)
Figure 2. XPS spectra of monometallic and bimetallic catalysts in the Cu 2p3/2 energy window for (a) Cu/TiO2, (b) Cu-Ni/TiO2, (c) Cu/Al2O3,
and (d) Cu-Ni/Al2O3, and in the Ni 2p3/2 energy window for (e) Ni/TiO2, (f) Cu-Ni/TiO2, (g) Ni/Al2O3, and (h) Cu-Ni/Al2O3. Metallic peaks
(Cu0 & Ni0) represented with blue colour and metal oxide peaks (CuO & NiO) peaks represented with green colour. Orange colour represents
Ni(OH)x peak.
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80
XPS Derived Cu:Ni (%)
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Cu(%) in Cu-Ni/TiO2
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Ni(%) in Cu-Ni/Al2O3
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Cu(%) in Cu-Ni/Al2O3
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Ni(%) in Cu-Ni/TiO2
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0
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30
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60
Ar Sputtering Time (min)
Figure 3. Relative Cu:Ni surface concentration (%) for pre-reduced Cu-Ni/TiO2 and CuNi/Al2O3 catalysts measured by XPS during depth profiling experiments as a function
or Ar sputtering time.
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TCD Signal (a.u.)
o
192 C
0.024
0.016
0.008
0.000
o
127 C
(a) Cu/TiO2
(b) Cu/Al2O3
o
450 C
0.024
0.016
0.008
0.000
o
250 C
(d) Ni/TiO2
o
500 C (c) Ni/Al O
2 3
o
350 C
o
0.024
0.016
0.008
0.000
180 C
o
133 C
o
o
211 C o 373 C
277 C
100
200
300
400
(e) Cu-Ni/Al2O3
(f) Cu-Ni/TiO2
500
600
700
o
Temperature C
and (f) Cu-Ni/TiO2.
0.05g of Cu(10%)/Al2O3
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80
60
(a)
(b)
(c)
FOL
THFOL
MF
MTHF
2 Pent diols
Furans
FF conv.
40
20
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o
o
o
o
o
180 C, 1h 220 C, 2h 240 C, 6h 240 C, 6h 240 C, 8h 220 C, 8h 220 C, 8h 220 C, 8h
25bar
H
15bar
H
25bar
H
35bar
H
35bar
H
15bar
H2 15bar H2
25bar H2
2
2
2
2
2
800
Figu
re 4.
H2 TPR
spect
ra
for
(a)
Cu/T
iO2,
(b)
Cu/
Al2O
3, (c)
Ni/T
iO2,
(d)
Ni/A
l2O3,
(e)
CuNi/A
l2O3
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Figure 5. FF conversion and product yields for Cu/A2O3 catalysts for the reaction conditions
shown along the x-axis at four catalyst loadings. All reactions except those noted by a, b, and
c were executed at a FF loading of 1 g, 25 mL IPA solvent, and Cu(10 wt%)/Al2O3 catalyst
loading of 0.05 g. The catalyst loadings for the other three were (a) 0.15 g for Cu(10
wt%)/Al2O3], (b) 0.3 g for Cu(10 wt%)/Al2O3, and (c) 0.3 g for Cu(25 wt%)/Al2O3.
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FOL
THFOL
MF
MTHF
1, 2 pentane diols
FF Conv.
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80 %
80
60
40
20
14.3%
0
1h
4h
Ni/TiO2
1h 4h
1h 8h
1h
Ni/Al2O3
Cu/TiO2
Cu/Al2O3
8h
Figure 6. FF conversion and product yields over monometallic Ni and Cu supported on
Al2O3 and TiO2 catalysts at different reaction times. All reactions were run with FF loading
of 1 g, catalyst loading of 0.3 g, 25 mL of 1,4 Dioxane as solvent, temperature of 200 oC, and
H2 pressure of 25 bar.
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Figur
e 7.
100
FF
80
80
conve
rsion
60
60
and
produ
40
40
ct
yields
20
20
as
functi
0
0
6
1
4
8
0.5
2
1
0.5
8
2
on of
Reaction Time (h)
Reaction Time (h)
reacti
on time over (a) Cu-Ni/TiO2 and (b) Cu-Ni/Al2O3 catalysts. All reactions were run at a FF loading of 1 g, catalyst loading of 0.3 g, 25 mL of 1,4
Dioxane as solvent, temperature of 200 oC, and H2 pressure of 25 bar.
100 (a)
FOL
THFOL
MF
1, 2 Pentane diol
FF Conv.
THFOL
MF
MF+MTHF
(b)
FOL
Conv./Yield (mol%)
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FOL
MF
FF Conv.
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(b)
FOL
MF
FF Conv.
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(c)
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MF
THFOL
FF Conv.
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20
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0
R1
R2
R3
R4
Number of Recycles
R5
0
40
20
R1
R2
R3
R4
R5
Number of Recycles
0
R1
R2
R3
R4
R5
Number of Recycles
Figure 8. FF conversion and product yields as a function of number of catalyst recycles, R, for (a) Cu/Al2O3, (b) Cu /TiO2, and (c) Cu-Ni/TiO2
catalysts. After R4, catalysts were calcined at 450 °C for 5 hours and reduced at 450 °C for 3 hours prior to R5. Reaction conditions were a FF
loading of 1 g, catalyst loading of 0.3 g, 25 mL of 1,4 Dioxane as solvent, temperature of 200 oC, H2 pressure of 25 bar, and 2 hour run time.
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100 (a)
DMF
MFF
HMF Conv.
100 (b)
FF Conv.
Others
80
40
MFF
60
HMF Conv.
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DMF
MF
60
40
DMF
Conv./Yield (mol%)
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0
0.5
1
2
4
6
FOL
MFF
0
8
0
Reaction Time (h)
2
4
6
8
Reaction Time (h)
Figure 9. (a) HMF and (b) HMF +FF conversion and product yields as a function of reaction time over the Cu-Ni/TiO2 catalyst. For (a),
reactions were run at a HMF loading of 0.5 g, catalyst loading of 0.3 g, 25 mL of 1,4 Dioxane as solvent, temperature of 200 oC, and H2 pressure
of 25 bar. For (b), reactions were run at a FF loading of 0.5 g, HMF loading of 0.25 g, catalyst loading of 0.3 g, 25 mL of 1,4 Dioxane as solvent,
temperature of 200 oC, and H2 pressure of 25 bar.
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Cu
Ni
Cu
OH
O
η2(C=O)
>>>>>
η1(O)
η1(O)
H2
O
O
H-H
H
H
Al2O3
E
Int, Ni-Al 2O3
≈E
TiO2
Int, Cu-Al 2O3
E
Int, Ni-TiO2
>E
Int, Cu-TiO2
Figure 10. Schematic diagram of the proposed operating states of Cu-Ni bimetallic catalysts on Al2O3 and TiO2. For Cu-Ni/Al2O3 significant
exposure of extended Ni domains drive efficient ring hydrogenation to form THFOL, whereas the segregated structure of Cu-Ni on TiO2
facilitate MF formation. EINT: Energy of interaction between metal (either Ni or Cu) and support (Al2O3 and TiO2).
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Scheme 1. Reaction pathway for FF conversion. FF = Furfural, FOL = Furfuryl alcohol, MF = Methyl furan, THFOL = Tetrahydrofurfuryl
alcohol, and MTHF = Methyl tetrahydrofuran.22
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Scheme 2. Reaction pathway for HDO of HMF to DMF. HMF = 5-Hydroxymethyl furfural, MFF = Methyl furfural, BHMF = Bis
(hydroxymethyl) furan, MFOL = Methyl furfuryl alcohol, and DMF = Dimethyl furan.2
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